Hi Help for daughter,
Welcome to the group

I'm glad you decided to jump in and join us. I have found this site and it members to be really helpful with negotiating my Partner's undiagnosed BPD ex-wife. I know you you will find the same with your mom.
It sounds like you are dealing with a double whammy...mom and sister. Do you have other siblings? Is your dad in the picture?
You are already using one of the best tools and that is setting boundaries, which is what you are doing by blocking calls until you are ready to talk. Boundaries are about protecting ourselves not punishing other people.
Guilt...you are not alone in the FOG (Fear, Obligation, Guilt) we all have/do experience that too. Being the child of someone with BPD can make the FOG really difficult because you can be conditioned to respond in ways that aren't always healthy. I've watched my partner's daughters wrestle with Guilt too when it comes to their mom. Sometimes it's about letting the discomfort of FOG just happen without doing anything...it's about getting comfortable with discomfort.
How did you find out about BPD? I discovered it by Googling "Chronic Lying" and there it was. I then hit the library and did a lot of reading on the subject. That really helped me get a good grounding in what it was, but it was coming here that gave me practical tools. Are you in Therapy at all? That can also be helpful.
Here are more links that you might find interesting/helpful
https://bpdfamily.com/message_board/index.php?topic=56281.msg518358#msg518358https://bpdfamily.com/message_board/index.php?topic=108307.msg1064890#msg1064890Again welcome

Panda39